Either a photocopy of your marriage certificate or a wedding invitation. I went for my honeymoon last year and they never asked - I asked them if they wanted to see it and they said "Nope, your travel agent already notified us it was your honeymoon".. works for me..- they don't always ask - but I'd bring it anyway - the perks are worth it!!

to answer the original question a wedding invitation works just show them at the front desk we used that at CSA last week. to answer southernanna's question we got a bottle of champagne delivered to the room and a free 25 minute massage for my wife and myself.